The development of inorganic films or membranes which are selectively permeable to specific gases and are able to withstand the adverse environments encountered in most processes is becoming increasingly important. Such membranes must be stable at high temperatures and resistant to chemical attack to be suitable for use in a combined process involving a catalytic reaction and product separation. Through the use of such selective permeation membranes, the yield of catalytic processes which are currently restricted by thermodynamic equilibrium can be significantly improved.
In U.S. Pat. No. 4,902,307, in which one of the inventors is also one of the inventors of this invention, previous art concerning inorganic membranes is reviewed. U.S. Pat. No. 4,902,307 is hereby incorporated herein by reference. In U.S. Pat. No. 4,902,307, a technique, called the "opposing reactants deposition technique" for depositing internal SiO.sub.2 layers within porous Vycor tubes ("sandwich configuration") is described. The deposition reaction set forth in the patent was the oxidation of silane (SiH.sub.4). Briefly, that technique required the flow of one reactant, silane, inside the support tube, and the other reactant, oxygen, outside the support tube. The reactants diffuse in opposite directions and meet at some intermediate region within the tube wall where the film deposition reaction takes place. Once all the open pore paths are blocked by the deposited SiO.sub.2, a barrier is formed which is highly selective to hydrogen permeation.
The thickness of the deposited film varies inversely with the reaction rate. Thus, increasing the temperature and the concentration of the reactants increases the reaction rate and results in thinner films. The practical temperature of SiO.sub.2 deposition by oxidation of silane was found to be in the range of from 400.degree. to 500.degree. C. Below this range the reaction rate was too slow, and the deposition film too thick; above this range silane would decompose thermally throughout the porous substrate forming a thick layer of silicon of extremely low permeability to all gases, including hydrogen.
The SiO.sub.2 films deposited at about 450.degree. C. had hydrogen permeation rate coefficients of about 0.2 cm.sup.3 /cm.sub.2 -min-atm, and H.sub.2 :N.sub.2 permeation rate ratios of about 3000, both measured at 450.degree. C. immediately after deposition. Subsequent exposure to high temperatures, especially in the presence of water vapor, caused the permeability to hydrogen to decrease considerably. For example, heating at 600.degree. C. for one day in the presence of water vapor decreased the permeation rate by a factor of 3, and heating at 700.degree. C. for an additional day led to a further 30% drop of that rate. This undesirable decrease in permeability seems to be caused by densification of the SiO.sub.2 film.